Astigmatism is optically characterized as the condition wherein one meridian of the image field is focused in front of an orthogonal meridian. In this manner, one meridian can be properly focused on the retina while the other is in front of or behind the retina. In such a case, the meridian focused in front of the retina could be described as myopic. Further, if one meridian were focused on the retina and the meridian ninety degrees away were focused behind the retina, the meridian focused behind the retina could be described as hyperopic. It is also possible to have one meridian focused in front of the retina and the other principle meridian focused behind the retina; respectively, the first could be described as myopic and the second hyperopic. This case is frequently described as mixed astigmatism.
Animal studies have demonstrated the ability to influence the development of refractive error by placing the peripheral image field in front of or behind the retina while the central image field is accurately focused. Corneal reshaping to reduce the amplitude of myopia, by increasing the radius of curvature of the central cornea while leaving the peripheral curvature unchanged, has demonstrated the ability to reduce or halt the progression of myopia. The mechanism is understood to be the placement of the peripheral image field in front of the retina while the central image field is accurately focused on the retina.
The current lens designs for corneal reshaping are intended to increase the radius of curvature of the central cornea for the treatment of myopia and decrease the radius of curvature of the central cornea for the treatment of hyperopia. There is evidence that, fortuitously, the lenses for reducing the amplitude of myopia also regulate the progression of myopia by way of the resultant off axis optical power of the mid peripheral cornea relative to the on axis central optical power of the cornea.
The prior art teaches the methods of reshaping the central cornea to correct refractive error. For the treatment of myopia, the central corneal radius is increased by way of applying a lens having a central radius of curvature longer than the pre treated central corneal radius; conversely, for the treatment of hyperopia, the central corneal radius is decreased by way of applying a lens having a central radius of curvature shorter than the pre treated central corneal radius. In the prior art, the mid peripheral cornea is found to remain substantially unchanged during treatment.
The prior art does not address mid peripheral corneal reshaping or off axis optics to place the peripheral image field in front of or behind the retina and while the central image field is accurately focused. As a result, the prior art is unable to effectively eliminate astigmatism or reduce astigmatism amplitude. The present invention addresses these needs and other limitations of the prior art.